Test results show drinking water is safe downstream of Duke Energy’s coal ash spill this week, but that doesn’t mean the Dan River is unscathed.

Scientists worry about the long-term effects of metals that are bound to the estimated 82,000 tons of ash dumped into the Dan.

The particles will fall into river sediment. But under the right conditions, those potentially toxic metals can mix with water and work their way up the food chain – from insects to fish to birds.

The metals will likely stay in sediment in fast-flowing areas of the river, said Dianne Reid, section chief of environmental sciences for the N.C. Division of Water Resources.

But if the deposits settle in deeper, slow-flowing water where oxygen levels drop, chemical changes could release metals into the water.

Duke University scientists watched that happen in the aftermath of the nation’s biggest ash spill, by the Tennessee Valley Authority in late 2008.

A year after 5.4 million cubic yards of TVA ash swamped 300 acres, the researchers sampled Tennessee’s Emory River. They found that large doses of arsenic and selenium, during low-oxygen conditions, were released into the water from river-bottom sediment.

“At the beginning we saw elevated levels of contaminants in surface water, but that diminished over time,” said Avner Vengosh, a Duke geochemist. “Then we started to find results from the pore water (in sediment) showing a high level of contaminants. They reached up to 2,000 parts per billion of arsenic.”

North Carolina’s surface water standard for arsenic is 50 ppb.

Duke’s spill, at 60,000 to 100,000 cubic yards, was tiny compared with TVA’s. But the effect on the Dan, Vengosh said, “I think it would be the same thing. ... When we see an accumulation of coal ash, it’s forever until you remove the coal ash.”

A 2011 study of lakes near North Carolina ash ponds found a similar pattern – low levels of metals in surface water, but higher readings in sediment.

It’s far too early to tell whether scientific findings will translate into actual environmental harm. On the Emory River, Tennessee officials say, it did not.

“We’ve not documented any long-term impacts to fish and wildlife,” said Debbie Duren of Tennessee’s Department of Environment and Conservation. Duren is preparing a natural resources damage assessment from the TVA spill.

“There were initial, obvious physical impacts from the amount of ash we had, smothering and things like that. But as far as the studies of toxicity in fish and wildlife, those have not shown a lot of impact.”

The department continues to monitor harmful elements that can accumulate in animal tissue, such as selenium, Duren said.

Selenium is a naturally occurring element that is found in coal ash and can cause deformities in young fish. Selenium dumped into a lake at Duke’s Belews Creek power plant, in Stokes County, wiped out 19 of the lake’s 20 fish species in the 1970s.

Chemistry aside, the deluge of ash into both rivers could smother worms, mussels and other river-bottom life.

“If the animal couldn’t move fast, it was smothered,” said Chuck Head, another Tennessee environmental official.

TVA cleanup a guide

After the Tennessee deluge, TVA dredged 3.4 million cubic yards of ash and silt from the Emory and shipped it by rail to a landfill in Alabama. The 15-month operation was complicated by efforts to disturb the riverbed as little as possible.

Ash that spilled into wetlands and coves was put back on the spill site, encircled this time by a 60-foot wall, and will be capped.

TVA expects the cleanup to be finished sometime in 2015, about seven years after the spill happened. It will cost the utility $1.1 billion to $1.2 billion.

North Carolina will look to the TVA spill as officials evaluate the prognosis for the Dan, Reid said.

“A lot of it’s going to depend on actually how much the coal ash is spread out along the river,” she said of the cleanup.

Roane County, where the TVA spill happened, found unexpected benefits amid its ash catastrophe.

“It’s a mixed bag, really,” said County Executive Ron Woody. “The long-term impact will probably be perception rather than actual damage.”

The accident inadvertently helped the local economy, he said. More than 1,200 workers poured in to work on the cleanup. Local communities got $43 million from TVA that helped build new schools and sewage-treatment plants.

“The negative we don’t know how long we’ll have is this stigma,” Woody said. “It’s a deterrent to us as we sell real estate moving forward. Perception is reality for folks, and if (buyers) don’t understand it, they see unknown risks.”